Ethylene adsorption on thin films of Ni, Pd, Pt, Cu, Au and Al; Work function measurements

The changes in work function φ upon adsorption of C2H4 on clean film surfaces of six fcc metals (Ni, Pd, Pt, Cu, Au and Al) have been followed by means of photoelectron emission at 293 K. A marked difference was observed in the behaviour between Ni, Pd and Al on the one side and on Cu, Au and Pt on the other side: while with Ni, Pd and Al, φ as a function of coverage goes through a maximum, with Cu, Au and Pt, φ only decreases. In the discussion, the data obtained by work function measurements are related to other literature data. Several films covered with C2H4species were also submitted to a heat treatment while in other experiments H2 was admitted to the surface covered by C2H4 species. In some experiments C2h4 was admitted to surfaces covered by H2. In all cases φ was measured. The experiments reveal that C2h4is absorbed only reversibly on Cu and Au. On Ni, Pd and Pt, C2H4 is adsorbed initially with dissociation and this leads to an increase in φ on Ni and Pd and a decrease on Pt. Hydrogenated reactive species contribute to the lowering of φ observed with Ni, Pd and Pt. As with Cu and Au also on Ni, Pd and Pt a weakly bound C2h4is observed which leads to a decrease in φ as well. The behaviour of φ indicates that upon Al, C2h4 adsorbs first dissociatively to a small extent, while the weakly bound C2H4species act as intermediates for strongly adsorbed species which were observed after some time.